Cargill's Castle Trust chairman Steven De Graaf said spray painted tags had become more prevalent at the category one historic building in the past six months.
Yesterday, he called in Graffiti Doctor (Otago) owner and operator Roger Knauf to assess whether the graffiti could be removed and at what cost.
But the castle's ''fragile'' walls could not be water blasted without causing further damage, Mr Knauf said.
Just a thin layer of sand plaster covered pebble concrete mix and it had eroded in parts, making the surface extremely delicate, he said.
Usually, a chemical solution was applied to paint, which was then removed by water blasting.
Mr De Graaf was loathe to paint over the tagging, as it would leave patches of fresh paint on the historic building's facade.
He did not want vandals further damaging the 1877 structure by adding to the graffiti and had contacted police.
Fines of up to $1000 could be imposed on anyone convicted of such vandalism, Mr Knauf said.
Neighbours also kept a close eye on the private property and reported anyone illegally on site, Mr De Graaf said.
''I want to knock it [tagging] on the head before the place is covered in it,'' he said.
Recent purple, red and blue graffiti was ''more offensive'' than the relatively subtle marks left by people etching their names in the stone, he said.
Many of those etchings were done in the 1970s and 1980s when the castle was neglected, he said.
The trust had limited funds and all its money was needed for a planned restoration.
''We have engaged a structural engineer who has done preliminary plans and is now working on more detailed drawings. The idea is to have a steel framework on the inside linking it all together, get a roof on it, to keep the weather out, and maybe put in perspex windows.
''With the steel framework in place people could visit the castle as a historic attraction and walk around the upper levels safely. We hope to be able to progress the restoration this year,'' he said. Information panels would also be installed, detailing the site's history.
Eventually, the trust hoped to link Cargill's Castle to Tunnel Beach and St Clair, via walking tracks.
Mr De Graaf said the castle attracted ''a lot'' of interest from tourists as well as wedding parties and photographers, but until it was structurally safe, the property had to be fenced off and closed to the public.